The current process for printing security documents, especially currency, is sheetfed intaglio. Sheetfed intaglio inks are based on oxidizable resins and alkyds and are very slow drying. Typically, one side of the currency is printed first and after 24-48 hours, the other side is printed. The typical printing speed of sheetfed intaglio is about 45-75 m/min. U.S. Pat. No. 4,966,628 discloses typical sheetfed intaglio inks suitable for printing of security documents.
The term "intaglio printing" refers to a printing process wherein a printing cylinder or printing plate carries the engraved pattern and the engraved recess is filled with printing ink to be transferred to the printing substrate in order to create the document. In this type of printing, typically a rotating engraved cylinder (usually manufactured from steel or nickel and plated with chromium) is supplied with ink by one or more template inking cylinders by which a pattern of inks of different color is transferred to the printing cylinder. Any excess of ink on the surface of the cylinder is then wiped off by a rotating wiper cylinder covered by a plastisol, using a dilute aqueous solution of sodium hydroxide as an emulsifying medium for the wiped-off excess ink. Thereafter, the printing pattern is transferred, under a pressure of up to 27 kg/mm.sup.2, to the substrate. The principal requirements for a security document heatset intaglio printing ink are as follows:
(a) correct rheological properties in respect to transfer of the ink to the printing cylinder and transfer therefrom to the substrate; PA1 (b) ability of the excess ink to be easily and quantitatively removed from the non-image areas of the die surface by the wiping cylinder (wipeability); PA1 (c) ease of cleaning the wiping cylinder by means of a dilute aqueous caustic soda solution containing about 1% of NaOH and 1% sulfonated castor oil; PA1 (d) stability of the ink on the printing rollers; i.e. control of the evaporation of volatile materials during the printing process; PA1 (e) film-forming characteristics allowing handling of the webs carrying printed films of up to 200.mu. thickness immediately after printing, PA1 (f) proper drying properties when printing at speeds of up to 200m/min. with engravings of up to 200.mu. thickness; PA1 (g) outstanding chemical and mechanical resistance of the printed document pursuant to specifications established by INTERPOL at the 5th International Conference on Currency and Counterfeiting in 1969 and by the U.S. Bureau of Engraving and Printing in BEP 88-214 (TN) .sctn.M5. PA1 A. a resin selected from the group consisting of: (i) an epoxy resin comprising the condensation product of bisphenol A and epichlorohydrin and represented by the formula wherein n=0 to 15: ##STR1## (ii) the reaction product of: (a) the ester obtained from the esterification of the epoxy resin of (i) with a drying oil fatty acid and (b) a mixture of 20-28% of one or more unsaturated monobasic acids having a polymerizable double bond and 80-72% of one or more reactive monomers having a polymerizable double bond; and (iii) mixtures of (i) and (ii); PA1 (a) chemical resistance to solvents, acids, alkalies, soaps and detergents; PA1 (b) ink film integrity rub test; PA1 (c) crumple test PA1 (d) laundering test PA1 (e) soiling test PA1 (f) humidity and temperature stability PA1 (g) flexing test PA1 (h) fade resistance test PA1 (i) magnetic properties test
All of the foregoing requirements have been met by the ink compositions of the present invention. Indeed, the ink compositions of the present invention have been accepted by the Bureau of Engraving and Printing for use in printing currency by heatset intaglio printing.